Suppose that there was a tribe of nomadic mermaids whose lives were primarily centered around following pods of dolphins to eat.

They live in the equivalent of the frigid antarctic seas. As they possess both lungs and gills, they can surface above the water or swim in the watery depths for hours on end without having to worry about drowning or suffocation. Their strong claws can easily chip through ice in a manner of minutes.

This is where the ice plays in.

In this setting, the mermaid cut floe or small icebergs away from the land and hollow holes in their sides. That is where they rest and temporarily store their food. I haven't made up a name for them yet, so I'll just call them trailers for the purposes of the question. Through the use of whale sinews, the trailers are tied to the titular pack animals, which transport the trailers to where the mermaids want them to go.

I'd rather not have them ride the currents, because they wouldn't be able to make sails, and also because they wouldn't be able to float away from assailants very quickly.

$\begingroup$social hunters and grazers... omnivores and herbivores. intelligent carnivores. it's rare for carnivores other than mammals to be social. some sharks do flock a bit.$\endgroup$
– com.prehensibleFeb 23 '18 at 2:35

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$\begingroup$To clarify: these pack animals are to pull small icebergs?$\endgroup$
– RonJohnFeb 23 '18 at 2:36

$\begingroup$Where does the weird musclerope come from? Being muscle, they're going to have to kill a lot of something in order to obtain enough to harness anything to an iceberg.$\endgroup$
– RonJohnFeb 23 '18 at 12:08

9 Answers
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You're going to want to obtain some whales. Not only are they the largest mammals alive, they also have the largest muscles.

Whales travel in pods, the equivalent of a "pack" of wolves. They're strong, and should be able to lug your iceberg-storage system around for you.

They also breathe air, so they can take advantage of the holes your mermaids cut in the ice.

Depending on the type of whale, they may not need you to provide a food source, as they could eat plankton while traveling.

Alternatively, if you pick grey whales, they don't eat very much during migration, so if you're traveling between their migration locations that might be a benefit.

If you choose Orcas, they can hunt their own food and provide defense against sharks at the same time (Orcas are known to eat manta rays, hammerhead sharks, seals, sea lions, and baleen whales).

I can't imagine your floating icebergs being heavier than a man made boat, but here's some footage of a whale pulling a boat. If your mermaids can cut the icebergs to have less drag that could be helpful as well.

$\begingroup$Herman Melville wrote a book about whales that's got some pretty useful whale facts and some depictions of whales pulling boats, too. Aside from the terrible storyline, it's actually a pretty good read.$\endgroup$
– Jakob LovernFeb 22 '18 at 23:47

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$\begingroup$Fed and well treated Orcas may be cooperative as we see them on water shows, but I wouldn't be so sure about wild ones... I just imagined a caravan of carnivorous elephants to illustrate this point. They may not only attack their owners when they start to dislike them, they'd also try to eat them when they were hungry.$\endgroup$
– CPHPythonFeb 23 '18 at 9:36

The Greenland shark is one of the largest living species of shark,
with dimensions comparable to those of the great white shark.
Greenland sharks grow to 6.4 m (21 ft) and 1,000 kg (2,200 lb), and
possibly up to 7.3 m (24 ft) and more than 1,400 kg (3,100 lb). Most
Greenland sharks observed have been around 2.44–4.8 m (8.0–15.7 ft)
long and weigh up to 400 kg (880 lb).

The Greenland shark is known to be a scavenger, and is attracted by
the smell of rotting meat in the water. The sharks have frequently
been observed gathering around fishing boats. It also scavenges on
seals.

Greenland sharks seem more like pack animals to me. Big, slow, tough, strong, stupid. They like it ice cold. They live for centuries, getting gradually bigger and bigger. They eat carrion and waste which should be easy for the merfolk to provide.

Tuna -- powerful swimmers, top speed nearly 40 mph, weight more than half a ton each, known to swim with dolphins already; warm-blooded (one of the only fish species that is), some species near the arctic (e.g. 60 degrees); maximum age 10-50 years (depending on species); vulnerable/threatened/endangered by human fishing.

I'd go with turtles. True, the RL ones are tropical; but in a fantasy setting already rife with mermaids, adding arctic turtles would be a minor edit. You can make them as large as you want, or have teams of smaller turtles (like huskies pulling sleds) harnessed to a single trailer---driving them would be a complex skill that the natives start learning as toddlers. Turtles have the disadvantage of needing air, but to have surface-traveling trailers you need open water anyway. And turtles can stay submerged for longer than dolphins or whales. They are non-aggressive and they eat jellyfish mostly, which are everywhere. And they have the ability to crawl onto land, albeit as clumsily as their mermaid owners.

Of RL animals, I would go with @Willk's Greenland shark, because it is a scavenger (easy to keep fed with scraps) and because it needs no air. It seems dangerous at first glance, but that's no worse than the huskies Inuits use. And like most sharks it has a super-keen sense of smell, which would be very valuable in tracking dolphins or whatever.

Finally, an off-the wall idea: giant squid. No need for sinew-rope harnesses---just tether the trailer to the pair of long tentacles, and away you go! It eats fish, but tends to be a deep diver---that behavior would have to be de-trained. Attach a skin air-bladder to the squid, like the Inuit did to harpooned game?

And BTW, the Vikings used leather sails, so there is no problem with sail material, really.

Most likely, whales. They live in pods with a leader, so they have the herd instinct of other domesticated pack animals such as a horse or cow. All land based animals who have been trained as pack animals have that herd instinct... they're used to taking orders, so transferring that from the pack leader to a human isn't that great a leap.

Whatever aquatic creature you choose, you do have to train it to haul packs where you want them, and they need a fair amount of endurance. Herbivores tend to have more endurance than predators with a pack instinct, who are more sprinters.

The less advanced creatures who don't have a herd or pack instinct, like sharks, probably won't make good pack animals for the same reason alligators aren't used as pack animals... they don't have the intellect, temperament or instinct for domestication. Not used to taking orders from a pack leader.

Also, alligators tend to eat anyone who tries to domesticate them, which could dampen enthusiasm for the endeavor.

I assume you are thinking of a storyline for a book. This mermaid-world is fictional,so why not invent a pack animal? I assume Nomadic Mermaids were inspired by the mermaids of legend. Those mermaids of legend were inspired by various forms of oceanic creatures,so the Nomadic Mermaids' pack animals can be inspired by real life animals. You will need to invent an animal which could live in the your antarctic equivalent condtions. Draw your inspiration from real oceanic animals. This will depend upon the similarity of your "Antarctic equivalent" to the real life conditions of antarctica.

The animal should be endothermic.You can count on endotherms.
It should,when in the wild untamed state, swim just below the water's surface.
It should be a social, easily domesicated animal.

The just-below-the surface behavior is the hardest part to invent. Consider the natural history of this animal. It needs to have exhibited a similar behavior,pre-domestication. Invent an ecosystem. This just below the surface swimming might be a feeding behavior. You have to work at this.

I think a whale-like endotherm that has historicaly eaten something from just under the ice floes would be a good candidate for your pack animals.